1. Field of the Invention
This invention relates generally to the field of magnetic storage, and more particularly to the field of magnetoresistive read heads.
2. Description of the Related Art
Magnetoresistive (“MR”) read heads for magnetic storage devices (e.g., disk drives) utilize MR sensors (e.g., giant magnetoresistive or “GMR” sensors) which comprise a ferromagnetic free layer having a magnetization orientation which can be switched between two states by applying a magnetic field. When reading data from magnetic media, magnetic fields from the data bits being read induce the magnetization orientation of the free layer to be in one of the two states. Since the resistance of the MR sensor depends on the magnetization orientation of the free layer, the magnetization state of the free layer can be detected by using a sensing current through the MR sensor to read the data.
Typically, the MR sensor is formed adjacent to a hard magnetic bias layer which generates a longitudinal magnetostatic bias field for various desirable results. For example, the bias field can provide magnetic stability against domain wall movement within the free layer, thereby reducing noise. In addition, the bias field can enhance the linearity of the MR sensor during the readback operation.
The bias layer typically comprises a nonmagnetic underlayer and a ferromagnetic hard bias (HB) layer. The underlayer serves to induce desirable morphology (e.g., in-plane crystallographic texture) in the HB layer. Exemplary materials for the underlayer include, but are not limited to, Cr, W, and CrTi alloys. The HB layer produces the longitudinal bias field which is applied to the free layer of the MR sensor. Exemplary materials for the HB layer include, but are not limited to, CoPt, CoCrPt, and other cobalt-based alloys. The remanence moment (Mr) and the thickness of the HB layer (t) are properly chosen whereby their product (Mr×t) provides a suitable bias field for the MR sensor. In addition, the HB layer preferably has a sufficiently high coercivity (Hc) for magnetic stability of the HB layer, and a preferred in-plane easy axis orientation of magnetization for producing uniform magnetic charge along the edges of the MR sensor to provide the bias field.
The bias field is strongly dependent on the properties of the HB layer (e.g., material, morphology) and on the junction profile between the bias layer and the free layer. Thus, the HB layer significantly impacts such properties of the MR sensor as amplitude, asymmetry, hysteresis, magnetic read width (“MRW”), skirt ratio, and pulse half-width (“PW50”). For example, the granular nature of the HB layer can cause non-uniformity of the bias field at the edges of the free layer. Such non-uniformities can be the source of varying performance properties among manufactured MR sensors, even among MR sensors formed on the same wafer (so-called “nearest neighbor jump,” or “NNJ”), which can vary by 30%-40%. These non-uniformities can be especially influential when dimensions of the MR sensor (e.g., the stripe height) are comparable to the length scale of the grain size of the HB layer.
Generally, a smaller grain size in the HB layer can reduce the non-uniformities due to grain size, as well as reducing the reader noise and further enhancing the signal-to-noise ratio of the MR sensor. However, a reduction of the grain sizes of the HB layer can result in a lower coercivity of the HB layer. It is therefore desirable to reduce the grain sizes of the HB layer while maintaining a high coercivity.